Lapping surface conditioning means



LAPPING SURFACE CONDITIONING MEANS Clifford Stead, North Smithfield, R.I., assignor to The Taft-Peirce Manufacturing Company, Woonsocket, R.I., a corporation of Rhode Island Application July 26, 1957, Serial No. 674,500

18 Claims. (Cl. 51-131) The present invention relates to lapping apparatus for truing surfaces of work pieces and, more particularly, to means for wear conditioning the lapping surface of a rotatable lap plate thereof.

A general object of the invention is to provide rotary lapping surface conditioning means which carries magnetic means effectively magnetically loading it toward the lapping surface of a rotatable lap plate thereby efficiently avoiding the provision of the conditioning means in a form of excessive bulk or with expensive and complicated supplemental mechanical loading means.

A more specific object of the invention is to provide such magnetically-loaded lapping surface conditioning means in a form characterized by a plurality of appreciably spaced localized Wearing surfaces for contact of the rotatable lapping surface which perform a more efficient conditioning action than does a conditioning ring having an annular zone of contact with the lapping surface.

Another object of the invention is to embody such magnetically-loaded conditioning means in a form which, in addition to providing appreciably spaced localized wearing surfaces, may serve effectively as a rotary cage for nested work pieces or for loosely carrying spider means in which such work pieces may be nested.

A further object of the invention is to provide such magnetically-loaded conditioning means in a reversible form which may be desirably loaded magnetically when positioned with one side up on the rotatable lapping surface and which may have different loading when turned over to a reversed position to give desired different lapping surface conditioning action in its two positions.

A still further object of the invention is the provision of structural embodiments of the invention which may be readily constructed and allow efi'icient use and operation thereof.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Fig. 1 is a plan view to reduced scale of the table and rotatable lap plate portions of a lapping machine which, by way of example, embodies a form of the lapping surface conditioning means of the present invention;

Fig. 2 is an enlarged sectional view, with parts shown in elevation and others broken away, taken substantially on line 2-2 of Fig. 1;

Fig. 3 is a detail section to still larger scale, with parts broken away, taken substantially on line 3-3 of Fig. 1;

Fig. 4 is a sectional detail similar to Fig. 3, the section being taken substantially on line 4-4 of Fig. 1; and

United States Patent ice Fig. 5 is an enlarged sectional view, with parts shown in elevation and others broken away, of a portion of the circular lap plate shown in Figs. 1 and 2 and a modified form of circular cage unit employed therewith in accordance with the present invention.

Referring to the drawing, in which like numerals identify similar parts throughout, it will be seen that, by way of example, the present invention may be embodied in a lapping machine which may comprise a frame structure 10 which includes a work resting edge means in the form of an annular frame ring plate 11 in which is mounted a rotatable circular lap plate 12 by any suitable means, such as its central drive shaft 13, as best seen in Fig. 2. Preferably, as there shown, the top lapping surface 14 of the lap plate 12 is disposed in a plane slightly above the top surface 15 of the frame ring 11 for clearance to avoid any difiiculty should the lapping surface conditioning means in its action bridge thereacross. Similar clearance preferably is provided in the plate top surface 14 in its axial zone by providing a central depression 114 therein, as shown in Fig. 2. Near the center of the lap plate 12 but eccentric thereof and preferably within the central depression 114 is mounted a stub shaft hub or boss 16 which rotatably carries an eccentric roller 17 mounted for free rotation thereon.

In the machine illustrated by way of example, the lapping surface conditioning means for truing the surface 14 of the lap plate 12 is in the form of a plurality of cage units 18-18 so constructed as to carry nested therein a plurality of work pieces, surfaces of which are to be trued by lapping on the lap plate 12. Since each of the cage units 18 is to be magnetically-loaded by magnetic means carried thereby, in accordance with the present invention, the rotatable horizontal lap plate 12 is formed of suitable paramagnetic material, such as cast iron, and each cage unit comprises or includes means to condition the lapping surface 14 in the form of a wear body.

Each wear body is in the form of lateral rotary wear frame means which may be an annular frame 19 supported upon a plurality of appreciably spaced depending legs 20-20 and -120 in any desired and suitable number, each having a localized bottom wear surface 21 or 121, with the wear surfaces thereof arranged substantially in a common plane. As indicated in Fig. 1, each frame annulus 19 may be supported, for example, by eight such circumferentially spaced legs 2020 and 120-120 with the circumferential edge 22 of the annulus bearing against the cylindrical side face of eccentric roller 17. The frame ring 11 supports a plurality of biasing structures 23-23, one for each wear body unit 18 and each biasing structure preferably comprises a lateral arm 24 supported by an offset boss or hub 25 and with its inner free end 26 carrying a freely rotatable roller 27 hearing against the circumferential edge 22 of the frame annulus 19. It will be seen from Fig. 1 that with the lap plate 12 rotating in the counterclockwise direction there indicated, an arc of the frame annulus 19 which is in extent appreciably less than extends between the point of contact of its circumferential edge 22 with the eccentric roller 17 and the point of contact thereof with the arm-supported roller 27 on the lead side, so as to localize the position of such rotary wear frame means during rotation of the lap plate. Thus the biasing structure 23 constitutes means for holding this frame member in a rotary manner in one locality with the bottom wear surfaces 21-21 and 121-121 of the legs 2020 and 120-120 contacting or bearing upon the plate lapping surface 14 in an area radially outward of the plate axis whereby plate rotation will induce rotation of the frame member in this locality.

In accordance with the present invention, loading means are provided for the rotary lateral frame means or annulus assasos name or cage unit 18 toward the lap plate 12 by magnetic attraction. In the illustrated; embodiment, every other one of the eight supporting legs 2)2l and l20l2fi is provided with such magnetic loading means so that there are four thereof.

Each magnetically-loaded leg 12% has spaced opposite magnet poles in its wear surface 121i juxtaposed to the plate lapping surface 14 so as to superpose a portion of the plate, indicated at 28 in Fig. 4, which will provide a paramagnetic path between the spaced opposite poles in the leg wear surface for conduction therebetween of magnetic lines of force. One such magnetic pole may be provided as a sleeve 29 of paramagnetic material, preferably in the form of a cylindrical casing sleeve of cast iron which encases a permanent magnet 31 preferably in the form of a cylinder. The upper top pole end 31 of the permanent magnet 30 and the paramagnetic encasing sleeve 29 are intervened by suitable paramagnetic means which provides for transfer therebetween of magnetic lines of force. Such magnetic transfer means preferably is in the form of a cylinder 32 of paramagnetic material, such as steel, which is fitted into the upper end of the casing sleeve 29 for abutment to the upper magnet pole end 31 and the inner face of the sleeve. The magnetic transfer cylinder 32 serves as a mounting hub having a top end extension 33 extending up through a hole 34 in the frame annulus l? for attaching the leg 120 thereto. The cylinder extension 33 may be anchored by any suitable means which may comprise a capping body 220 having a socket 35 into which the upper end 33 of the cylinder 32 extends and with the latter having an internally-threaded bore as receiving externally-threaded shank 37 of a fillister machine screw 38, with head 39 of the latter seated in a socket 40 in the upper end of the capping body.

The cast iron casing sleeve 29 is lined with a housing of diamagnetic material, preferably in the form of a brass sleeve 41, telescoped up into the lower end of the casing sleeve. The brass sleeve 41 intervenes at least the other or bottom pole end 42 of the permanent magnet 3t and cast iron casing sleeve 29 so as to prevent transfer of magnetic lines of force therebetween. As shown in Pig. 4, such housing sleeve 41 preferably extends up along the sides of the permanent magnet cylinder 30 to the bottom end of the hub cylinder 32 so that the magnet is completely housed therein. The other magnetic pole is in the form of a paramagnetic body, preferably another cylinder 43 which may be of cast iron, housed in the lower end of housing sleeve 41 and juxtaposed to the bottom pole end 42 of the magnet with the housing sleeve interposed between this pole cylinder and the casing sleeve 29 to prevent direct lateral transfer of magnetic lines of force therebetween. As will be seen from Fig. 4, the bottom ends of the casing sleeve 29, the housing sleeve 4-1 and the pole cylinder 43 provide substantially flat faces which are substantially aligned in a common plane and constitute the wear surface 121 of the magnetically-loaded leg 120.

As will be seen from Fig. 4, magnetic lines of force, which are illustrated by the arrows, may emanate from the upper end 31 of the permanent magnet St! to be trans ferred by the hub cylinder 32 across to the casing cylinder 29 to be carried by the latter to the superposed portion 28 of the lap plate 12, with the latter serving to transfer these magnetic lines of force inward to the pole cylinder 43 for transfer by the latter up to the lower other pole end 452 of the permanent magnet. Thus each leg 120 is magnetically-loaded toward or against the lapping surface 14- of the lap plate 12.

As will be seen from Fig. 3, each of thelegs 20, which is not magnetically-loaded is in the form of a cylindrical body 44 having an upwardly extending neck 1330f reduced diameter extending up through one of the plurality of holes 34-34 in annulus 19 and serving as a hub for such leg. Like capping body 220 and fillister screw 38 secure the hub neck 133 to the frame annulus ll in the manner described for attachment of each of the magneti Cally-loaded legs 12% In conditioning the lapping surface 14 of the rotatable lap plate 12 by such a wear body unit 1%, the structure so far described is similar to that shown in the centrally open annulus 19 at the lower left portion of Fig. 1. With the lap plate 12 rotating in a counterclockwise direction, the annulus 19 supported on the lapping surface 14 'by its plurality of depending legs 202li and 3.20 12b is rotated in its localized position, as dictated by the central concentric roller 17 and its biasing roller 27, also in a countel-clockwise direction so as to sweep the bottom wear surfaces 2i2l and 121-421 of the supporting legs 20-20 and -3120 circularly across the lapping surface 14. Also due to the eccentric mountingof the central roller 17 with respect to the axis of the lap plate 12 at the point 45, indicated in Fig. l, the wear body annulus 19 will be given eccentric rotary motion during its rotation so as to assure an efiicient wearing action.

As may be preferred, the rotary lateral frame means which constitutes the magnetically-loaded wear structure will serve as a cage for a plurality of the work pieces to be trued by the lapping surface 14 and, for this purpose, it is provided with a plurality of spaced holes in which a plurality of the work pieces may be nested for resting upon the plate lapping surface. Although such structure to provide a plurality of the spaced nesting holes may be in the form of a transverse web made integral with the frame annulus 19 closing up the central portion thereof so that the two together are in the form of a disc, as proposet in Fig. 5, preferably a separate spider 46 in the form of a circular plate structure which may be made from resinbonded fiber, such as Masonite, loosely located in the centrally-open wear frame structure down between the inner sides of the circularly-arranged legs 2920 and 120-420, is provided for this purpose. The spider 46 has a plurality of the work piece nesting holes 47 47 porvided therein, as will be seen in the upper left hand position of another one of the cage units 13. As illustrated in the cage unit structure 18 in the third position at the right hand side of Fig. 1, each of the holes il-t7 may carry therein in loosely nested fashion a work piece 48 to have the bottom surface thereof suitably trued by the lapping surface 14 of the lap plate 12 when the latter is rotated.

It will be seen from Fig. 2 that the legs 20-20 and 120-120 are of appreciably greater length than the thickness of the spider 46 so that a limited arc of the circumferential edge 49 of the latter will contact in rotation of the cage unit 18 the inner sides of the legs. Rotation of annulus 19 and its carrying legs 20-20 and 120-120 does not induce rotation of the spider 46 and therewith the work pieces 48-48 carried thereby since the spider will be rotated by the rotating plate surface 14-even if the cage annulus is held stationary toprevent it from rotating. The annulus 19 serves only as'a frame member for tying together in relative positions the annularlyarranged circumferentially-spaced wear legs 20-20 and l20120, the localized bottom surfaces 21-21 and 121121 of which alone provide the truing surfaces which contact the lapping surface 14 of the lap plate 12 and it has been found that such annularly-spaced localized wear surfaces perform a most efiicient truing action when suitably loaded.

By providing the cap bodies 220-220 in the forms illustrated in Figs. 2, 3 and 4, which in general overall shape and size may be similar to the legs 20-20 and 120-120, they may serve as a second set of wear legs when, the annular frame or, cage structure 18 isturned .might be said to fall therebetween.

assasos over with their top end surfaces 221-221 resting upon the lapping surface 14 of the lap plate 12. In such a case, the frame or cage structure 18 has no magnetic loading with the loading being provided solely by the weight of the structure. It will be understood that a variance of magnetic loading may be attained by provid- 120-120 alternated on one side thereof with four un-.

loaded legs 2020, the opposite side may be provided with less than four magnetically-loaded legs similar to 120, such as by simply substituting the leg structure of Fig. 4 for some of the leg structures of Fig. 3 but reversed in mounted position with the cap bodies 220-220 of the latter being substituted for some of the magneticallyunloaded legs 20-20. In such embodiment of the invention, each wear body is provided in the form of frame means having one set of wear legs extending from the bottom side thereof and including legs which are magnetically-loaded, the structure having another set of wear legs extending in the opposite direction from the top side of the frame means and having less or no magnetic loading, the two sets of wear legs being alternately restable upon the plate lapping surface when the frame means is turned over.

The rotatable lap plate 12 of the lapping table may have its lapping surface 14 in any conventional form or design, the conditioning means hereof being useful for truing a variety of types of lapping surfaces. For example, the lapping surface may be of the smooth uninterrupted type, or it may have grooves therein to receive and partially contain the lapping abrasive and the carrier liquid used therewith and to minimize development of vacuum sticking of work piece faces to the lapping surface. Such a grooved lap may have radial grooves, a helical groove or grooves in a grid pattern it its lapping surface, and the like.

As previously proposed and as is indicated in Fig. 5, the means of each cage unit, indicated at 118, in which a plurality of the spaced nesting holes may be provided, may be in the form of a transverse web 146 made integral with the frame annulus 119 by means of a connecting wall or annular zone 246 so that the central portion of the cage is closed up by such web with the structure being in the form of a shaped disk construction carrying at radially spaced intervals about the periphery thereof legs similar to those indicated at 120 and 220 in Figs. 2 and 4. Obviously the central web 146 may, if desired, be made of greater thickness so that its top surface is in the plane of the top of the peripheral edge of ring 119, so that such combined ring and spider web have a fiat circular top face, and with such disk being centrally thickened so that its bottom face between the supporting legs 120 is disposed in the vicinity of the lap surface 114 to permit nesting in the spider holes 47-47.

work pieces 48-48 of thicknesses appreciably less than the lengths of the cage legs 120.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language,

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In lapping apparatus for truing surfaces of work pieces including a rotatable lap plate of paramagnetic material, means to condition the lapping surface of said plate comprising, in combination, a circular wear body adapted to have surface contact with the plate lapping surface in an annular zone and in rotation of the topping surface to move with different circular motion thereagainst for truing the latter, and loading means for said body comprising magnetic means serving as a source of magnetic lines of force carried by and uniformly distributed annularly about said body for rotation therewith with magnetic lines of force emanating from said body having portions of their conducting paths provided by the adjacent paramagnetic material of said lap plate to press said wear body uniformly against the lapping surface during relative rotation.

2. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, means to condition the lapping surface of said plate comprising, in combination, lateral rotary circular wear means having in an annular zone thereof contact areas rested upon the lapping surface radially outward of the axis of rotation thereof, means localizing the position of said wear means while permitting rotation thereof against the lapping surface for conditioning the latter, and loading means carried by said wear means for rotation therewith comprising magnetic means serving as a source of magnetic lines of force carried by and uniformly distributed in the annular zone of said wear means with magnetic lines of force emanating from said source having portions of their conducting paths provided by the adjacent paramagnetic material of said lap plate to press said wear means uniformly against the lapping surface during relative rotation.

3. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, means to condition the lapping surface of said plate comprising, in combination, rotary lateral frame means, a plurality of appreciably spaced legs supporting said frame on the plate lapping surface and having localized bottom wear surfaces for conditioning the plate lapping surface in rotation of said frame means, permanent magnet means carried by at least some of said legs for loading said frame toward said plate by magnetic attraction, and means for holding said frame means in a rotary manner in one locality radially outward of the plate axis for rotation there upon rotation of the lap plate.

4. The lapping surface conditioning means defined in claim 3 characterized by each magnetically-loaded leg having spaced opposite magnetic poles in its wear surface juxtaposed to the plate lapping surface with magnetic lines of force passing through a portion of the plate between said poles.

5. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of para magnetic material, means to condition the lapping surface of said plate and carry the work pieces thereon comprising, in combination, rotary lateral frame means having a plurality of spaced holes in which a plurality of work pieces may be nested for resting upon the plate lapping surface to be lapped thereby, a plurality of appreciably spaced legs supporting said frame on the plate lapping surface and having localized bottom wear surfaces for conditioning the plate lapping surface in rotation of said frame means, permanent magnet means carried by at least some of said legs for loading said frame means toward said plate by magnetic attraction, and means for holding said frame means in a rotary manner in one locality radially outward of the plate axis for rotation there upon rotation of the lap plate.

6. In lapping apparatus for truing surfaces of work I pieces including-a rotatable horizontal lap plate of paramagnetic material, means to condition the lapping surface of said plate and carry the work pieces thereon comprising, in combination, rotary lateral frame means having a plurality of spaced holes in which a plurality of 1 tion of said frame means, permanent magnet means carried by at least some of said legs for loading said frame means toward said plate by magnetic attraction, each leg carrying a permanent magnet having spaced opposite magnetic poles in its wear surface juxtaposed to the plate lapping surface with magnetic lines of force passing between said poles through a portion of the plate therebeneath, and means for holding said frame means in a rotary manner in one locality radially outward of the plate axis for rotation there upon rotation of the lap plate.

7. In lapping-apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, means to condition the lapping surface of said plate and carry the work pieces thereon comprising, in combination, a rotary annular frame member, a plurality of legs depending from said member and spaced appreciably circumferentially thereof with said legs supporting said frame appreciably above the plate lapping surface, said legs having localized bottom wear surfaces for conditioning the plate lapping surface in rotation of said frame member, means for holding said frame member in a rotary manner in one locality with the bottom wear surfaces of said legs contacting the plate lapping surface in an area radially outward of the plate axis whereby plate rotation induces rotation of said frame member in said locality, permanent magnet means carried by at least some of said legs for loading said frame member toward said plate by magnetic attraction, and a circular spider plate loosely located between said legs below said frame member for free rotation therebetween with said spider plate having a plurality of spaced holes in which a plurality of work pieces may be nested for resting upon the plate lapping surface to be lapped thereby.

8. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, means to condition the lapping surface of said plate and carry the work pieces thereon comprising, in combination, a rotary annular frame member, a plurality of legs depending from said member and spaced appreciably circumferentially thereof with said legs supporting said frame appreciably above the plate framernember toward said plate by magnetic attraction, each leg carrying a permanent magnet having spaced opposite magnetic poles in its wear surface juxtaposed to the plate lapping surface with magnetic lines of force passing between said poles through a portion of the plate therebeneath, and a circular spider plate loosely located between said legs below said frame member for free rotation therebetween with said spider plate having a plurality of spaced holes in which a plurality of work pieces may be nested for resting upon the plate lapping surface to be lapped thereby.

9. The lapping surface conditioning means defined in claim 8 characterized by each;magnetically-loaded leg having one of said poles in the form of a sleeve of paraarlnagnetic, materialmhousingrits permanent, magnet, in the upper end thereof, paramagnetic means interveningone end of said magnet and said sleeve to provide for transfer therebetween of magnetic lines of force, a housing of diamagnetic material telescoped into the lower end of said paramagnetic sleeve and intervening the latter and the other end of said magnet, and the other pole being in the form of a paramagnetic body housed in the lower end of said diamagnetic housing and spaced by the latter from said paramagnetic sleeve with the bottom faces of the latter and said body being substantially aligned in a common plane and constituting the wear surfaces of said le 10. The lap-ping surface conditioning means as defined in claim 8 characterized by each magnetically-loaded leg having one of said poles in the form of a cylindrical casing sleeve of paramagnetic material having the paramagnetic intervening transfer means in the form of a magnetic transfer cylinder fitted into the upper end thereof with said cylinder serving as a mounting hub attaching said sleeve to said annular frame member, a housing sleeve of diamagnetic material telescoped into the bottom end of said casing sleeve below said cylinder, a cylindrical permanent magnet coaxially housed in said housing sleeve having lower and upper pole ends with the latter abutting said cylinder, and the other pole being in the form of another cylinder of paramagnetic material housed in the bottom end of said housing sleeve and abutting the lower pole end of said magnet with said housing sleeve intervening said casing sleeve and said pole cylinder, the bottom ends of said casing sleeve and pole cylinder having substantially flat faces substantially aligned in a common plane and constituting the wear surfaces ofsaid leg.

11. The lapping surface conditioning means as defined in claim 5 characterized by two sets of said wear legs with one set extending from the bottom side of said frame means and including some of those which are magnetically loaded, the other set of said wear legs extending in the opposite direction from the top side of said frame means alternately to be rested upon the plate lapping surface when said frame means is turned over and with the second set of said legs having less magnetic loading.

12. The lapping surface conditioning means as defined in claim 11 characterized by said frame means having no magnetic loading in the set of legs on one side thereof, the magnetic loading of some of said legs being confined to the set on the other side thereof.

13. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, rotary means to condition the plate lapping surface comprising a rotary annular frame to be rested loosely on the lapping surface, a set of legs depending from one side of said frame at circumferentially spaced points, each of said legs having a localized bottom wear surface for conditioning the plate lapping surface in rotation of said frame thereon, magnet means carried by at least some of said legs for loading said frame toward said plate by magnetic attraction, and another set of legs extending from the opposite side of said frame at circumferentially spaced points, each of the latter set of legs also having a localized end wear surface alternately for conditioning the plate lapping surface in rotation of the frame thereon when turned over.

14. The lapping surface conditioning means defined in claim 13 characterized by each leg carrying magnet means having spaced opposite magnetic poles in its wear surface for juxtaposition to the plate lapping surface whereby a superposed portion of the latter provides a paramagnetic path between said poles for conduction therebetween of magnetic lines of force.

15. The lapping surface conditioning means asadefined in claim 14 characterized by said magnet means for each magnetically-loaded leg being in the form of a permanent magnet with such leg having one of said poles in the form of a sleeve of paramagnetic material housing its permanentmagnet in the upper end thereof, paramagnetic means intervening oneend. of: said, magnet and said sleeve to provide for transfer therebetween of magnetic lines of force, a housing of diamagnetic material telescoped into the lower end of said paramagnetic sleeve and intervening the latter and the other end of said magnet, and the other pole being in the form of a paramagnetic body housed in the lower end of said diamagnetic housing adjacent the last-mentioned end of said magnet and spaced by said housing from said paramagnetic sleeve with the bottom faces of the latter and said body being substantially aligned in a common plane and constituting the wear surfaces of said leg.

16. The lapping surface conditioning means as defined in claim 14 characterized by said magnet means for'each magnetically-loaded leg being in the form of a permanent magnet with such leg having one of said poles inthe form of a cylindrical casing sleeve of paramagnetic material having a magnetic transfer cylinder fitted into the upper end thereof with said cylinder serving as a mounting hub attaching said sleeve to said annular frame member, a housing sleeve of diamagnetic material telesco'ped into the bottom end of said casing sleeve below said cylinder, a cylindrical permanent magnet coaxially housed in said housing sleeve having lower and upper pole ends with the latter abutting said cylinder, and the other pole being in the form of another cylinder of paramagnetic material housed in the bottom end of said housing sleeve and abutting the lower pole end of said magnet with, said housing sleeve intervening said casing sleeve and said pole cylinder, the bottom ends of said casing sleeve and pole cylinder having substantially flat faces substantially aligned in a common plane and constituting the wear surfaces of said leg.

17. In lapping apparatus for truing surfaces of work pieces includinga rotatable horizontal lap plate of paramagnetic material, rotary means to condition the plate lapping surface comprising a rotary annular frame which is centrally open for loose nesting therein of a work piece-carrying spider to be rested loosely on the lapping surface, a set of legs depending from one side of said frame at circurn'ferentially spaced points, with said legs being of appreciable length wholly to receive such spider therebetween, each of said legs having a localized bottom wear surface for'conditioning the plate lapping surface in rotation of said frame thereon, at least some of said legs being magnetically loaded by permanent magnet means having a pair of opposite poles, each magnetically-loaded leg having one of said poles in the form of a casing sleeve of paramagnetic material encasing its permanent magnet in the upper end thereof, paramagnetic means intervening one end of said magnet and said sleeve to provide for transfer therebetween of magnetic lines of force, a housing of diamagnetic material telescoped into the lower end of said casing sleeve and intervening the latter and the other end of said magnet, and the other pole being in the form of a paramagnetic body housed in the lower end of said diamagnetic housing and spaced by the latter from said casing sleeve with the bottom faces of the latter and said body being substantially aligned in a common plane and constituting the wear surfaces of said leg.

18. In lapping apparatus for truing surfaces of work pieces including a rotatable horizontal lap plate of paramagnetic material, rotary means to condition the plate lapping surface comprising a rotary annular frame which is centrally open for loose nesting therein of a work piece-carrying spider to be rested loosely on the lapping surface, a set of legs depending from one side of said frame at circumferentially spaced points, with said legs being of appreciable length wholly to receive such spider therebetween, each of said legs having a localized bottom wear surface for conditioning the plate lapping surface in rotation of said frame thereon, at least some of said legs being magnetically loaded by permanent magnet means having a pair of opposite poles, each magnetical ly-loaded leg having one of said poles in the form of a cylindrical casing sleeve of paramagnetic material having a magnetic transfer cylinder fitted into the upper end thereof with said cylinder serving as a mounting hub attaching said sleeve to said annular frame member, a housing sleeve of diamagnetic material telescoped into the bottom end of said casing sleeve below said cylinder, a cylindrical permanent magnet coaxially housed in said housing sleeve having lower and upper pole ends with the latter abutting said cylinder, and the other pole being aligned in a common plane and constituting the wear surfaces of said leg.

References Cited in the file of this patent UNITED STATES PATENTS 426,671 Macbeth Apr. 29, 1890 2,056,182 Gardner Oct. 6, 1936 2,180,952 Gattoni Nov. 21, 1939 2,772,521 Voytech Dec. 4, 1956 2,782,571 Hanson Feb. 26, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,883,803 April 28, 1959 Clifford Stead It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 44; for porvideid" read provided column 5, line 60, for face 114" read face 14 column 6, line 8, for "topping read lapping Signed and sealed this 25th day of August 1959.

S ttest:

KARL H. AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents 

